Fuel filters are necessary components used to protect combustion engines by filtering out contaminants and thereby preventing damage to the engine and other downstream components such as valves, fuel injectors, fuel lines and related components. Fuel can entrain a wide variety of contaminants from a number of sources. Frequently fuel oxidizes and forms resinous materials such as asphaltenes commonly referred to as varnishes. Bacteria can grow in fuel, particularly fuel that contains moisture. The growing bacteria forms a "slime" on inner surfaces of fuel tanks, fuel lines and fuel filters. The slime often sloughs off the surfaces and fuel transports the free slime through the fuel lines. The fuel filter filters out the varnishes and bacteria; however, in the process the varnish and bacteria block the fuel filter element. The blocked filter exhibits a reduced fuel flow and increases the pressure differential across the filter element, thereby reducing engine performance and fuel efficiency. To maintain engine performance and reliability, the fuel filters must be replaced, often as frequently as every 2,000 to 4,000 vehicle miles. For many vehicles, particularly commercial trucks that travel hundreds or even thousands of miles a month, this significantly increases vehicle maintenance and operating costs and can reduce reliability.
Fortunately, the formation of varnishes and bacteria growth can be inhibited by adding fuel additives such as antioxidants and biocides to the fuel. Dispersants can be added to break up and dissolve some of the existing material clogging the filters and other fuel related components. To be effective, the additives must be constantly maintained in the fuel. This is often difficult. Unlike other fluid systems, such as the coolant and oil systems, the fuel system does not re-circulate all of the fluid. Therefore, the additives must be continuously added to the fuel at a controlled rate.
It is difficult to maintain a constant or desired level of an additive in the fuel. Typically an operator adds a bottled additive to the vehicle fuel tank with each fuel fill-up. While many bottled fuel additives are commercially available, often operators do not consistently add the additive with each fill-up. The additive might not be readily available or the operator may forget to include the additive. When the additive is added to the fuel tank, it does not always form a homogeneous mixture with the fuel. Fuel tanks do not include reliable methods for mixing fuel. Generally operators rely upon the turbulence created during a fill-up and by vehicle motion to mix the additive and fuel. Furthermore, the additive concentration in the fuel may vary, depending upon the amount of fuel added--assuming a set amount of additive is added with each fill-up.
The following references provide a background for filters in general that contain additives.
Davis in U.S. Pat. No. 5,372,942 describes a pressurized fuel filter having an additive embedded in wax; the wax/additive composition dissolves when contacted with fuel. Additional additive is only added when a portion of the fuel becomes clogged and the fuel level in the filter rises to contact and dissolve additional amounts of the wax/additive composition.
Lefebvre in U.S. Pat. No. 5,591,330 discloses an oil filter that includes a thermoplastic material having oil oxidation and acidification additives. The hot oil recirculating through the engine and the filter dissolves that thermoplastic material thereby releasing the additives into the oil.
Hudgens in U.S. Pat. No. 5,662,799 discloses a coolant filter for recirculating coolant. The coolant filter includes a supplemental coolant additive (SCA) and a release means to maintain the concentration of SCA already dissolved in the recirculating coolant fluid.
One of the design challenges addressed by the present invention is how to control the release of fuel additive into the fuel. The present invention solves this challenge in several ways, each of which is believed to be a novel and unobvious solution. The solution avoids the undesirable side effects, which have already been described and can be exploited to increase the maintenance interval, increase the fuel filter life span and enhance engine performance and fuel efficiency.